Underwater adhesion has several potential medical, household, and industrial applications. Underwater adhesives have been prepared by methods that typically rely on in situ polymerization, covalent crosslinking, or the use of highly specialized biological or biomimetic polymers. There has been a focus in the literature on biomimetic materials inspired by sessile marine organisms such as zebra mussels, tubeworms, and barnacles. Currently used strategies include the use of extracted or recombinant proteins, reactive dopamine derivatives, or coacervation and subsequent crosslinking of custom-designed polymers, which mimic the molecular structure and, consequently, the underwater adhesion properties of natural adhesive proteins. Some biomimetic adhesion strategies have included mixing polyelectrolytes with catechol-based crosslinkers, which convert the polymer solutions into adhesive gels. Other strategies involve synthesizing polyelectrolytes with crosslinkable catechol-based sidechain groups, or designing biomimetic polyelectrolyte complexes that undergo coacervation followed by either ionic or covalent crosslinker (which converts the liquid coacervates into adhesive solids).
Despite the current strategies, many challenges have limited the yield and application of biomimetic adhesives. The use of biological and biomimetic adhesion remains somewhat limited by the high cost of natural proteins, inefficient recombinant protein production, complicated syntheses, the use of potentially harmful oxidants to induce crosslinking, and the need for highly-specialized polymer structures. Similarly, older adhesion strategies such as epoxides, acrylic adhesives, and cyanoacrylate gels, suffer from deficiencies such as relying on chemical reactions to set the adhesive, being limited to a specific set of adhesion substrates, forming underwater bonds that are permanent and/or not self-healing upon failure, or requiring in situ polymerization. There is a need in the art for additional and improved underwater adhesives.